This invention relates generally to turbine engine shrouds disposed about rotating articles and to their assemblies about rotating blades. More particularly, it relates to air cooled gas turbine engine shroud segments and to shroud assemblies, for example for use in the turbine section of a gas turbine engine, especially segments made of a low ductility material.
Typically in a gas turbine engine, a plurality of stationary shroud segments are assembled circumferentially about an axial flow engine axis and radially outwardly about rotating blading members, for example about turbine blades, to define a part of the radial outer flowpath boundary over the blades. In addition, the assembly of shroud segments is mounted in an engine axially between such axially adjacent engine members as nozzles and/or engine frames. As has been described in various forms in the gas turbine engine art, it is desirable to avoid leakage of shroud segment cooling air radially inwardly and engine flowpath fluid radially outwardly through separations between circumferentially adjacent shroud segments and between axially adjacent engine members. It is well known that such undesirable leakage can reduce turbine engine operating efficiency. Some current seal designs and assemblies include sealing members disposed in slots in shroud segments. Typical forms of current shrouds often have slots along circumferential and/or axial edges to retain thin metal strips sometimes called spline seals. During operation, such spline seals are free to move radially to be pressure loaded at the slot edges, generally by radially outer cooling air, and thus to minimize shroud segment to segment leakage. Because of the usual slot configuration, stresses are generated at relatively sharp edges. However as discussed below, current metallic materials from which the shroud segments are made can accommodate such stresses without detriment to the shroud segment. Examples of U.S. patents relating to turbine engine shrouds and such shroud sealing include U.S. Pat. Nos. 3,798,899xe2x80x94Hill; 3,807,891xe2x80x94McDow et al.; 5,071,313xe2x80x94Nichols; 5,074,748xe2x80x94Hagle; 5,127,793xe2x80x94Walker et al.; and 5,562,408xe2x80x94Proctor et al.
Metallic type materials currently and typically used to make shrouds and shroud segments have mechanical properties including strength and ductility sufficiently high to enable the shrouds to receive and retain currently used inter-segment leaf or spline seals in slots in the shroud segments without resulting in damage to the shroud segment during engine operation. Generally such slots conveniently are manufactured to include relatively sharp corners or relatively deep recesses that can result in locations of stress concentrations, sometimes referred to as stress risers. That kind of assembly can result in the application of a substantial compressive force to the shroud segments during engine operation. If such segments are made of typical high temperature alloys currently used in gas turbine engines, the alloy structure can easily withstand and accommodate such compressive forces without damage to the segment. However, if the shroud segment is made of a low ductility, relatively brittle material, such compressive loading can result in fracture or other detrimental damage to the segment during engine operation.
Current gas turbine engine development has suggested, for use in higher temperature applications such as shroud segments and other components, certain materials having a higher temperature capability than the metallic type materials currently in use. However such materials, forms of which are referred to commercially as a ceramic matrix composite (CMC) or monolithic ceramic materials, have mechanical properties that must be considered during design and application of an article such as a shroud segment. For example, CMC and monolithic ceramic type materials have relatively low tensile ductility or low strain to failure when compared with metallic materials. Therefore, if a CMC or monolithic ceramic type of shroud segment is manufactured with features such as relatively sharp corners or deep recesses to receive and hold a fluid seal, such features can act as detrimental stress risers. Tensile forces developed at such stress risers in that type segment material can be sufficient to cause failure of the segment.
Generally, commercially available CMC materials include a ceramic type fiber for example SiC, forms of which are coated with a compliant material such as BN. The fibers are carried in a ceramic type matrix, one form of which is SiC. Forms of monolithic ceramic materials, not reinforced with fibers, include SiC and SiN3. Typically, those types of materials have a room temperature tensile ductility of no greater than about 1%, herein used to define and mean a low ductility material. For example, CMC type materials generally have a room temperature tensile ductility in the range of about 0.4-0.7%. This is compared with metallic materials currently used as shrouds, and supporting structure or hanger materials, that have a room temperature tensile ductility of at least about 5%, for example in the range of about 5-15%. Shroud segments made from CMC or monolithic ceramic type materials, although having certain higher temperature capabilities than those of a metallic type material, cannot tolerate the above described and currently used type of compressive forces generated in slots or recesses for fluid seals.
One typical form of a gas turbine engine includes a circumferential array of shroud segments disposed circumferentially about and spaced radially outwardly from tips of a plurality or stage of rotating blades to enable the blades to rotate freely inwardly from the shroud segments. During engine operation, as blade tips intermittently pass the radially inner surface of the shroud segments, variations in pressure forces tend to move or vibrate the segments axially inwardly and outwardly. When a shroud segment is made of a low ductility material, it is desirable to avoid sealing circumferentially extending separations between axially adjacent engine members in a manner that results in a stress riser, as discussed above. Therefore, it would be advantageous to dispose on or at a radially outer surface of the shroud segment bridging the separation a spline or leaf seal member that is, or is capable of becoming, flat or planar in juxtaposition with, or is forced to conform with, a radially outer surface of the shroud segment bridging the separation.
The radially inner surface of a shroud segment is arcuate circumferentially to cooperate in spaced-apart juxtaposition with inwardly rotating blades. Conveniently, such shroud segment generally is made with a radially outer surface that is generally arcuate. Therefore, the above-described variable pressure induced radial movement of the shroud segment during engine operation is particularly significant at the axial edge portions of the shroud segment at which such a bridging seal would be disposed. Disposition of a flat or planar seal surface on a surface that is other than flat or planar results in a point or axial line contact between such cooperating members, enhancing vibration and or stress concentration at or along such contact. Therefore, a shroud segment and assembly of shroud segments configured to receive and hold a circumferentially extending fluid seal at an axial edge portion of a shroud segment without generating detrimental stress or vibration at a point or line contact can enable advantageous use of low ductility shroud segments with fluid seals retained between axially adjacent engine members without resulting in operating damage to the brittle shroud segments.
The present invention, in one form, provides a shroud segment for use in a turbine engine shroud assembly comprising a plurality of circumferentially disposed shroud segments. Each shroud segment comprises a shroud segment body including a circumferentially arcuate radially inner surface defining a circumferential arc, and a radially outer surface. The radially outer surface extends between a first, axially forward, outer surface edge portion and a second, axially aft, outer surface edge portion axially spaced apart from the first outer surface edge portion. At least one of the axially spaced apart outer surface edge portions comprises a surface depression portion extending circumferentially across the outer surface edge portion and including a planar seal surface. The planar seal surface is spaced apart radially outwardly from the circumferential arc of the segment body radially inner surface, defining a spaced-apart chord of the circumferential arc. The planar seal surface is joined with the shroud body radially outer surface through an arcuate transition surface.
In a turbine engine shroud assembly comprising a plurality of circumferentially disposed shroud segments as described above, at least one of the first and second axially spaced apart outer surface edge portions is distinct axially from a surface of an axially juxtaposed adjacent engine member by a circumferential separation therebetween. A fluid seal member, including a fluid seal member surface that is planar or formable to planar, is retained in the surface depression and extends circumferentially along and bridges the separation. The fluid seal member surface that is planar or formable to planar is in juxtaposition for contact with the planar surface depression portion of the shroud segment body along the separation.